Water at a temperature/pressure exceeding the critical point (specifically, water at a temperature/pressure exceeding 374° C./22.1 MPa) is known as supercritical water, and is capable of dissolving a huge variety of materials. Water in this supercritical state exists in a non-condensable, high density gaseous state, and is capable of completely dissolving non-polar or very slightly polar materials (such as hydrocarbon compounds or gases) which display only very limited solubility in water at room temperature, and it is reported that by also adding oxygen to the supercritical water, these dissolved materials can be oxidized and decomposed.
The organic toxic materials used in chemical weapons and the like are no exception, and can be dissolved completely in supercritical water, and by also incorporating dissolved oxygen in the supercritical water and reacting the organic toxic materials contained within the chemical weapons or the like in the supercritical water, oxidation and decomposition into non-toxic materials such as carbon dioxide, water, sulfuric acid and phosphoric acid can be achieved. For example, VX gas can be oxidized and decomposed into sulfuric acid and phosphoric acid, and GB gas can be oxidized and decomposed into hydrofluoric acid and phosphoric acid. Accordingly, in recent years in the U.S.A., tests have been conducted on using supercritical water in the disposal of chemical weapons that contain VX gas, GB (sarin) gas, mustard gas or the like, by decomposing and oxidizing, and thus detoxifying, the organic toxic materials of VX gas, GB (sarin) gas and mustard gas, which are difficult to break down under normal conditions. Once this method for decomposing, oxidizing and detoxifying the organic toxic materials of VX gas, GB (sarin) gas and mustard gas and the like using supercritical water becomes established, it will provide a much more environmentally friendly process than the conventional incineration treatment methods, as the supercritical water and oxidizing agent have no adverse effects on the environmental. Furthermore, because supercritical water is highly reactive, organic toxic materials such as VX gas, GB (sarin) gas and mustard gas can be decomposed, oxidized and detoxified within a short period of time. In addition, the decomposition treatment can be carried out within a closed system, meaning there is no danger of environmental pollution caused by emissions or discharge.
Furthermore, organic toxic materials such as PCBs and dioxin, which represent industrial waste products for which disposal is difficult, are also no exception, and can be dissolved completely in supercritical water. By adding oxygen and reacting the organic toxic materials within the supercritical water, oxidation and decomposition into non-toxic materials such as carbon dioxide, water, and hydrochloric acid can be achieved. This process can be carried out within a closed system, meaning that compared with conventional incineration treatment methods, there is no danger of environmental pollution caused by emissions or discharge.
When supercritical water is used as the reaction solvent for decomposing and oxidizing organic toxic materials such as VX gas, GB (sarin) gas and mustard gas, the oxidation and decomposition in high temperature, high pressure (400° C. to 650° C., 22.1 MPa to 80 MPa) supercritical water generates a mixture of inorganic acids such as sulfuric acid and phosphoric acid with a high concentration of oxygen. As a result, in order to enable supercritical water to be used as the reaction solvent for decomposing, oxidizing, and detoxifying organic toxic materials such as VX gas, GB (sarin) gas and mustard gas, the process reaction apparatus in the system used for detoxifying these organic toxic materials, and in particular the material used for producing the process reaction vessel, must display good corrosion resistance relative to this type of supercritical water environment containing inorganic acids.
Furthermore, when supercritical water is used as the reaction solvent for decomposing and oxidizing organic toxic materials such as PCBs and dioxin, the oxidation and decomposition in high temperature, high pressure (400° C. to 650° C., 22.1 MPa to 80 MPa) supercritical water generates a mixture of inorganic acids containing chlorine such as hydrochloric acid together with a high concentration of oxygen. As a result, in order to enable supercritical water to be used as the reaction solvent for decomposing, oxidizing, and detoxifying organic toxic materials such as PCBs and dioxin, the material used for producing the process reaction vessel in the system used for detoxifying these organic toxic materials must display good corrosion resistance relative to this type of supercritical water environment containing inorganic acids.
Consequently, Ni based corrosion resistant alloys, which are widely known as being highly resistant to corrosion, have been proposed as one possibility for a metal material that could be used for the process reaction apparatus used with supercritical water. Specific examples of such Ni based corrosion resistant alloys include Inconel (a registered trademark) 625 (as prescribed in ASTM UNS N06625, with a composition, expressed as weight percentages, that comprises, for example, Cr: 21.0%, Mo: 8.4%, Nb+Ta: 3.6%, Fe: 3.8%, Co: 0.6%, Ti: 0.2%, and Mn: 0.2%, with the remainder being Ni and unavoidable impurities), and Hastelloy (a registered trademark) C-276 (as prescribed in ASTM UNS N10276, with a composition that comprises, for example, Cr: 15.5%, Mo: 16.1%, W: 3.7%, Fe: 5.7%, Co: 0.5%, and Mn: 0.5%, with the remainder being Ni and unavoidable impurities). Recent reports have stated that Ni based alloys with even higher Cr contents display even better corrosion resistance relative to supercritical water containing inorganic acids. As a result, high Cr content Ni alloys such as MC alloy (with a composition comprising Cr: 44.1%, Mo: 1.0%, Mn: 0.2%, and Fe: 0.1%, with the remainder being Ni and unavoidable impurities) and Hastelloy G-30 (as prescribed in ASTM UNS N06030, with a composition that comprises, for example, Cr: 28.7%, Mo: 5.0%, Mn: 1.1%, Fe: 14.6%, Cu: 1.8%, W: 2.6%, and Co: 1.87%, with the remainder being Ni and unavoidable impurities) are now attracting considerable attention as potential materials for reaction apparatus.
However, amongst conventional Ni based alloys, Inconel 625 and Hastelloy C-276 do not provide adequate corrosion resistance to supercritical water containing acids such as sulfuric acid, phosphoric acid and hydrofluoric acid, and consequently if either of these materials is employed in a process reaction apparatus in a system used for detoxifying organic toxic materials, particularly if employed as the material for producing the process reaction vessel, then long term operation of the system is impossible. MC alloy on the other hand displays good initial corrosion resistance to supercritical water containing acids such as sulfuric acid, phosphoric acid and hydrofluoric acid. However, because the phase stability of the alloy is not entirely satisfactory, phase transformation tends to occur at the operating temperature, leading to a deterioration in the corrosion resistance. Consequently, if MC alloy is used in a reaction apparatus, then long term operation of the system is impossible.
Furthermore Inconel 625 and Hastelloy C-276 do not provide adequate corrosion resistance, with pitting occurring at the contact surfaces between the alloy and the supercritical water containing hydrochloric acid. As a result, if either of these materials is employed as the material for producing the process reaction vessel in a system used for detoxifying these types of organic toxic materials, then long term operation of the system is impossible. MC alloy on the other hand displays good initial corrosion resistance to supercritical water containing hydrochloric acid. However, because the phase stability of the alloy is not entirely satisfactory, phase transformation tends to occur at the operating temperature, leading to a deterioration in the corrosion resistance. Consequently, if MC alloy is used in a reaction apparatus, then long term operation of the system is impossible.
In addition, when a reaction vessel or piping is produced using Inconel (a registered trademark) 625, Hastelloy (a registered trademark) C-276 or Hastelloy (a registered trademark) G-30, then following manufacturing into a sheet or a pipe to make the process material, this process material must be subjected to further manufacturing process such as rolling or bending to complete the production of the reaction vessel or piping for the process reaction apparatus. Because a reaction vessel or piping produced in this manner is prepared by manufacturing process, internal stress or internal distortions remain within the product. Amongst conventional Ni based corrosion resistant alloys, it is known that Inconel 625 and Hastelloy C-276 develop stress corrosion cracking in contact with supercritical water containing non-chlorine based inorganic acids such as sulfuric acid, phosphoric acid and hydrofluoric acid. Consequently, if Inconel 625 or Hastelloy C-276 is used as the material for producing the reaction vessel or piping within a system for detoxifying organic toxic materials, then long term operation of the system is impossible. Hastelloy (a registered trademark) G-30 on the other hand displays good initial resistance to stress corrosion cracking when exposed to supercritical water containing acids such as sulfuric acid, phosphoric acid and hydrofluoric acid. However, because the phase stability of the alloy is not entirely satisfactory, phase transformation tends to progress gradually at the operating temperature (400° C. to 650° C.). If a stress field such as that generated by a high temperature, high pressure supercritical water environment is generated once this phase transformation has already progressed significantly, then stress corrosion cracking can occur. Consequently, Hastelloy G-30 is not an ideal material for producing a process reaction apparatus capable of long term operation.
Similarly, if conventional Ni based corrosion resistant alloys such as Inconel 625 and Hastelloy C-276 with residual internal stress or internal distortion are brought into contact with supercritical water containing hydrochloric acid or the like, then stress corrosion cracking occurs. Consequently, if either of these alloys is used for producing the reaction vessel or piping in a process reaction apparatus for detoxifying organic toxic materials, then long term operation of the system is impossible. Hastelloy (a registered trademark) G-30 on the other hand displays no stress corrosion cracking during initial operations with supercritical water containing hydrochloric acid. However, because the phase stability of the alloy is not entirely satisfactory, phase transformation tends to progress gradually at the operating temperature (400° C. to 650° C.). If a stress field such as that generated by a high temperature, high pressure supercritical water environment is generated once this phase transformation has already progressed significantly, then stress corrosion cracking can occur. Consequently, Hastelloy (a registered trademark) G-30 is not an ideal material for producing a process reaction apparatus capable of long term operation.